The present invention relates to an electrorheological gel comprising a curable silicone polymer, electrorheologically active particles, and a metal catalyst which result in the formation of a filled gel. The present invention further relates to a method for the preparation of the novel electrorheological gels.
When certain polarizable solid particles are dispersed in an electrically non-conducting hydrophobic liquid, the resulting suspensions exhibit peculiar rheological properties under the influence of an electrical field. These systems show a dramatic increase in viscosity and modulus with applied voltage, in some cases literally being transformed from a liquid to a virtual solid upon the application of the electric field. This change is reversible and typically takes place in a matter of milliseconds. Materials which exhibit this phenomenon are called electrorheological (ER) or electroviscous (EV) fluids, and have been known for at least the last fifty years. These fluids find utility in such areas as torque transfer and mechanical damping applications.
The early ER fluids comprised such systems as starch dispersed in transformer oil or silica gel dispersed in kerosine or mineral oil. Since these early discoveries, only a relatively small number of new systems, and improvements over old ones have emerged in this art.
Electrorheological (ER) fluids are composed of a polarizable solid phase dispersed in a dielectric fluid phase. ER fluids are unique in that they have the ability to change their characteristics from liquid-like to solid-like upon application of an external voltage. This change is reversible which means that the liquid-like state returns upon removal of the electric field. Upon application of a voltage, the solid particles form fibril-like networks which bridge the electrode gap. At this point, the material will not behave as a Newtonian fluid, but will exhibit a Bingham plastic behavior. Fluids exhibiting the Bingham plastic effect require application of a particular level of force (yield stress) before the material will flow again.
ER fluids employing silicone oil as the base fluid phase have also been disclosed. For example, Goossens et. al., in U.S. Pat. No. 4,645,614, teaches an electroviscous suspension which is based on a mixture of aqueous silica gel with silicone oil as the liquid phase to which a dispersant is added. The dispersant consists of amino, hydroxy, acetoxy, or alkoxy functional polysiloxanes having a molecular weight above 800. The electroviscous suspensions are disclosed as being highly compatible with elastomeric materials, non-sedimenting, non-flammable and physiologically acceptable. They are also described as heat and freeze resistant over a wide temperature range and are largely unaffected by temperature and pressure in their viscosity. Goossens et. al. in U.S. Pat. No. 4,668,417 discloses electroviscous fluids which comprise more than 25 weight percent silica gel having an H.sub.2 O content of 1 to 15 weight percent dispersed in 1 to 30 weight percent (based on the weight of the H.sub.2 O containing silica gel) of a non-conductive oil phase containing a soluble polymer having a molecular weight of 5000 to 1,000,000 and contains 0.1 to 10 weight percent of nitrogen compounds such as amines, amides, imides, or nitriles, or OH containing compounds such as alcohols, and 25-83 weight percent of C.sub.4 to C24 alkyl groups. It is further disclosed that these fluids have little or no thixotropic character and undergo little or no phase separation when left to stand and are readily dispersible if phase separation occurs.
Electrorheological fluid compositions having gel-like properties were described in Japanese Patent Application Laid-Open (Kokai or Unexamined) No. 04089893 which discloses an electrorheological fluid consisting of silica particles dispersed in an electrically insulating medium. The electroviscous fluid is taught as containing 10 to 50 percent by volume of spherical particles prepared by hydrolyzing a silicon alkoxide of the formula Si(OR).sub.4 where R is an alkyl group in the presence of an alkali catalyst and drying at a temperature of up to 500.degree. C. This publication further discloses that without a voltage applied the fluid shows good fluidity, while it becomes highly viscous or gel-like reversibly when applied with a voltage. Other such compositions were described in a journal article by Shiga et. al. entitled "Electroviscoelastic effect of polymer gel containing fine particles" (Chemical Abstracts 114:103279z, 1991) which discloses a silicone gel prepared by heating its preoligomer mixed with fine particles of Co(II) polymethacrylic acid salt having a small amount of adsorbed water. Shiga et. al. further disclosed that the electroviscoelastic effect of the silicone gel was larger than that of a suspension of the above particles in a silicone oil.
Moisture curable silicones have been disclosed. For example, Flackett et. al. in U.S. Pat. No. 4,546,017 discloses a sealant composition curable to an elastomer in the presence of moisture obtained by mixing a polydiorganosiloxane having terminal silicon-bonded hydroxyl groups, a defined complex of titanium, and an oxime silane crosslinking agent. The compositions may also contain conventional ingredients such as fillers, curing catalysts, and polydimethylsiloxanes having terminal triorganosiloxy groups. Letoffe et. al., in U.S. Pat. No. 4,824,924 discloses a method for the preparation of a diorganopolysiloxane having polyalkoxy end groups comprising reacting at least one alpha, omegadihydroxydiorganopolysiloxane polymer with at least one polyalkoxysilane in the presence of a catalytically effective amount of at least one organic oxime devoid of silicon. Letoffe et. al. further discloses that the resulting functionalized oils are well adapted for the formulation of single-component, storage-stable organopolysiloxane cold vulcanizable elastomeric compositions.
Other moisture-curable silicone compositions are disclosed in Popa et. al., in U.S. Pat. No. 5,162,460, which discloses a composition consisting essentially of a tetrafunctional or hexafunctional silicone polymer which is modified with a liquid organohydrogensiloxane such that when the functional groups are alkoxy radicals the organohydrogensiloxane contains at least 4 silicon hydride groups per molecule and when the functional groups are oxime groups, the organohydrogensiloxane contains at least 5 silicon hydride groups per molecule, with the proviso that when the functional groups are alkoxy radicals, the silicone composition further comprises an effective amount of a cure catalyst.
In contrast, the present invention relates to an electrorheological gel comprising a curable silicone polymer, electrorheologically active particles, and a metal catalyst which result in the formation of a filled gel which allows for large variations in the storage modulus of the material with the application of an electric field.